Some years ago, when I would get a new syringe of solder paste, there would always be an expiration date on it after which it was not to be used. Lately I have not seen that.I assume that the reason was that the fluxes in the paste lost their activity after a period of time. Do the newer solder pastes no longer "expire"? Or, is there an accepted standard life after which the paste should not be used? This darn stuff is getting so expensive that it is painful to have to throw it away when only a few percent of the container has been used.Can someone tell me whether the newer pastes are usable "indefinitely" or, maybe, suggest a product with a longer shelf life for those of us who only use it occasionally.Thanks,Tom

I have seen tins of solder paste turn to brown crumbly masses in the stockpile at work. Don't know how old they were, but I would suspect at least 10-15 years. Perhaps some volatile substance has evaporated from them, or maybe application of enough heat would melt it back into a smooth paste. I guess anything that contains components that can solidify or evaporate can have a shelf life? If in a syringe, the possibility of the ingredients hardening over extended time would presumably make it impossible to extrude from the tip. Maybe a solution is that liquid flux. I just don't use that, as it is messy if it gets out of hand. THe more baffling thing to me is expiration dates on solder. Yes, they have those.

The shelf life has mostly to do with meeting the manufacturer's specifications as it relates to thickness, solvents, and flux. Over time solvents will evaporate, causing a change in density. The metal particles will begin to settle. So the manufacturer will set a date before when they know these things will normally happen so they can guarantee a consistent result with their product. Paste used for bench use can be very old and still work fine but when you're applying it with a stencil and running hundreds or thousands of boards an hour, even the slightest difference in paste characteristics can cause errant solder balls, bridges, tombstoning, poor wetting and other sundry SMT soldering defects. Even just a change in ambient temperature when the paste is applied can affect how successful the reflow profile will be. So it really doesn't go "bad", but if too old it can be unpredictable in automated assembly processes.

I have several tubes of various solder alloy pastes I've had upwards of 20 years I use for occasional work. I just give the stuff a stir and when flowed manually with an iron or a hot air reflow they work just fine.

Most of the early electrical solder fluxes were based on organic compounds. Many used tree resins. Mixed with other chemicals, of course. It was not uncommon to have a flux start growing bacteria and mold, which fed on the organic compounds in the flux. This could sometimes appear as crystallization, separation, and one in particular I remember started to smell rather foul.

I am using solder paste that is 20 years old. Put the lid on tight and it will last forever.

Yes indeed. Exposure to the atmosphere, humidity, air components such as Oxygen, is the factor that provides the bacterias, molds, fungus spores the third ingredient needed to start eatin' the stuff...

are we talking about paste flux, as you would use to improve solder flow on surfaces, or are we talking about the kind of solder paste that is composed of tiny balls of solder in a paste medium, used for surface mount soldering? If the latter, which is what I thought the original poster was asking about due to the mention of it coming in syringes, I have found that it does get dry and much harder to work with, in fact almost unusable with stencils, when it's old. Which is too bad given how little one usually needs for hobby projects and how much you have to buy.

Are we talking about the rosin paste or the solder/rosin mix that is sold in syringes or tiny, thumb sized containers?

Regular old-fashioned rosin (like in a shoe polish tin) seems to last forever.

the solder/rosin mixture (a gray substance) does seem to become unworkable after a few years. I think it is a problem where the solvent carrier eventually dries out.

We used to take the liquid rosin (sold in bottles) and add in a bit of solvent so it would flow better. Each workstation had a tiny squeeze bottle with a dull needle tip. We used that when clipping out old components so you could use the iron tip to just flick the component leads right out of the circuit board.

Even that Kester stuff would solidify in a few months. and get white solids in the bottom of the bottle.

The paste solder in syringes (tiny balls of solder in paste for surface mount) has about a 6-month shelf life. That's what the sellers claim but the stuff really becomes unworkable after maybe 3 or 4 months even if kept refrigerated. Others have already commented that the old tins of rosin paste seem to last forever. Mine has to be 20 years old and is still fine.

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